The present invention is related to an MRI (magnetic resonance imaging) system and an MR imaging method both of which make use of the magnetic resonance phenomenon of nuclear spins present in a subject""s body, in particular, to the MRI system and the MR imaging method for conducting an ASL (Arterial Spin Labeling) method capable of providing images of perfusion (blood flows in tissue) or blood vessels.
Especially, in the present invention, the present inventor made an invention on the ASL method, which should be refereed to as an ASTAR (modified STAR using Asymmetric Inversion slabs) method based on a STAR (Signal Targeting Alternating Radio frequency) technique known as one ASL method.
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins of a subject placed in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and obtaining images using FID (free-induction decay) signals or echo signals induced with the excitation.
One category of the magnetic resonance imaging is ASL (Arterial Spin Labeling) imaging. This imaging provides perfusion (tissue blood) images in which blood vessels and microcirculation of a subject are reflected, without injecting contrast medium into the subject, i.e., with non-invasiveness.
This ASL method includes a xe2x80x9ccontinuous ASL (CASL) techniquexe2x80x9d and a xe2x80x9cdynamic ASL (DASL) technique.xe2x80x9d The CASL technique is a way of applying a largely continuous adiabatic RF wave, while the DASL technique is that of applying a pulsed adiabatic RF wave that can easily be practiced by a clinical MRI system.
The DASL technique includes two main techniques of STAR (Signal Targeting with Alternating Radio frequency) and FAIR (Flow sensitive alternating Inversion Recovery). These two techniques are further deformed into the following various modes.
The STAR technique is a way of imaging one-way flow (normally, an inflow direction of the arteries) with the use of a tagging RF pulse spatially offset from an imaging plane, as proposed by xe2x80x9cNishimura et al., MRM 7:472-484 (1988)xe2x80x9d and xe2x80x9cEdelman et al., MRM 31:233-238 (1994).xe2x80x9d However, in this case, owing to differences in MT (magnetization transfer) effects which will be caused by the tagging RF pulse, there occur signal errors of larger scales than the flow. Particularly, since the tissue blood flow is imaged based on flow signal components provided by differences in minute signals corresponding to orders that are 2% or less of the original signal, differences in the MT effects have a large influence on it.
A way to eliminate the differences in the MT effects is proposed by xe2x80x9cEdelman et al., Radiology, 192, 513-520(1994),xe2x80x9d which is an imaging method known as an EPISTAR (echo-planar imaging and signal targeting with alternating radio frequency) technique. In this technique, to try to eliminate differences in the MT effects, RF pulses symmetric in both thickness and offset amount are applied to the upstream and downstream locations of blood flows (artery flows) passing through an imaging plane, respectively. This application enables differences in the MT effects in the imaging plane to be eliminated or lowered, but, like the FAIR technique, resulting in that the blood flows in both the inflow directions into the imaging plane are imaged as well. Thus the effect of prohibiting the veins from being imaged, which is known as vein suppression, will be lost.
Meanwhile, compared to the STAR technique, the FAIR (flow-sensitive alternation inversion recovery) technique proposed by xe2x80x9cKwong et al., MRM 34, 878-887 (1995),xe2x80x9d for instance, hardly occur differences in the MT effects, thus lessening a transit delay time, because on-resonance IR pulses are employed as controlling/tagging RF pulses. However, because it is not impossible to separate blood inflow directions into an imaging slice, there arises a problem that using this technique alone cannot accomplish the vein suppression. In addition, when an inflow direction of a dominant blood vessel into a region of interest is desired to be determined, this is also impossible.
In the ASL technique, it is therefore important that the conflicting problems of canceling the MT effects and imaging only one-way flow are solved. A manner of solving or improving these two problems is proposed as, for example, techniques of xe2x80x9cNew EPI-STARxe2x80x9d and xe2x80x9cASI-STAR.xe2x80x9d Of these, the New EPI-STAR technique is described by Mai et al., ISMRM 1998, p1205,xe2x80x9d for example. In this technique, the nature of an adiabatic pulse is utilized to apply an IR pulse of 360 degrees to a tagging side and to apply two IR pulses of 180 degrees to the identical location in the controlling side to that in the tagging side, so that the MT effects are cancelled. This way corresponds to an improved EPISTAR technique, which allows flows from the tagging side to be imaged and enables multislice imaging.
Moreover, the ASI-STAR technique is considered an improved FAIR technique. This technique is performed such that a non-selective IR pulse is applied to a thickened region and a tagging pulse causes a larger offset in an inflow side. This allows the vein inflow side to be located approximately by a selective IR pulse.
However, the foregoing New EPI-STAR technique and ASI-STAR technique have drawbacks that will be described below. Where the New EPI-STAR technique is used to conduct single-slice imaging, two 180-degree pulses consecutively applied cannot provide a completely restored longitudinal magnetization Mz in the controlling-side region. Compared to a condition under which no pulse is applied, influence of an incomplete cancellation of MT effects appears as an amount that cannot be ignored and the RF power increases, leading to a larger SAR. In cases where multislice imaging is conducted, it basically increases a transit delay time, which is unfavorable to quantification.
On one hand, in imaging with the ASI-STAR technique, a frequency offset is given for only one side region. Hence, even if an amount of the offset is small, a difference in the MT effects between the controlling and tagging applications is left as an amount that is small, but cannot be canceled well, which cannot therefore be ignored completely. This amount becomes a major error factor in detecting blood that flows slowly. Moreover, as to the profiles of both vein-side tagging and controlling slabs, their slopes do not coincide with each other completely or approximately completely. As a result, veins that flow at slower speeds are excited, making it impossible to completely cancel the differences between the tagging and controlling applications.
The present invention is made to consider the foregoing problems caused by the prior art techniques. An object of the present invention is to provide highly quantified perfusion images or blood flow images (MRA) in which not only MT effects in an imaging region are mutually canceled steadily so as to lower difference errors due to signals from stationary tissue but also sensitivity is given to only one-way blood flows so as to extremely reduce the influence of veins, for example, a flow component composed of almost arteries being produced, without largely raising the power of RF waves and/or increasing SAR (RF exposure) excessively.
The present invention is to provide a technique for obtaining perfusion (tissue blood flows) or blood flow images (MRA) on the basis of the ASL technique; those images are non-invasively provided with no contrast medium injected.
In order to accomplish the foregoing objects, the present invention adopts two types of imaging techniques both belonging to the ASL method. One is an approach on a novel ASL method, which is referred to as an ASTAR (Signal Targeting with Alternated Radio frequency using Asymmetric Inversion Slab) technique by the present inventor, while the other one is an approach on novel signal processing conducted with the foregoing EPISTAR method.
1. Approach on ASTAR Technique
First, the ASTAR technique will be now be described.
The ASTAR technique according to the present invention can be applied to either the PASL technique using pulsed adiabatic RF waves or the CASL technique using large continuous adiabatic RF waves. The ASTAR on the PASL technique will be first described, then that on the CASL technique will follow.
1.1. ASTAR on PASL Technique
(Outline of ASTAR Technique)
FIG. 1 shows a positional relationship of slabs (or slices set spatially on the ASTAR technique on the basis of the PASL technique. In the figure, the lateral axis is assigned to the body axis direction z of a subject to be imaged, whilst the longitudinal axis at the center of an imaging slab in the z-axis direction is assigned to offset amounts of a modulation frequency. Two oblique dashed lines represent intensities of IR (inversion recovery) gradients.
According to this ASTAR (on PASL) technique, as shown in FIG. 1, an imaging slab is selectively set as an imaging region, and both a tagging slab (or Tag-IR slab) produced by an applied tagging IR pulse (for inversion) and a controlling slab (or Control-IR slab) produced by an applied controlling IR pulse is selectively set to the imaging slab.
Then, a first scan using a first pulse sequence composed of a train of pulses including a tagging IR pulse to be applied slice-selectively to the tagging slab and an imaging pulse train to be applied slice-selectively to the imaging slab (hereinafter, this scanning is referred to as a tagging (labeling) scan) and a second scan using a second pulse sequence composed of a train of pulses including a controlling IR pulse to be applied slice-selectively to the controlling slab and an imaging pulse train to be applied slice-selectively to the imaging slice (hereinafter, this scanning is referred to as a controlling scan) are performed sequentially in time in an appropriate order. An imaging mode for the tagging scan is called tagging mode, while that for the controlling scan is called controlling mode.
In performing the tagging and controlling scans, one characteristic is that, with offset frequencies of both the tagging and controlling IR pulses measured from the center of the imaging slab made to agree to each other, the slab thickness and positional offset of each imaging pulse are changed by the same scale factor. This allows the distances between the tagging-imaging slabs and the controlling-imaging slabs to be adjusted, thus making it equal or approximately equal to each other MT effects caused in the imaging slab by the application of both the IR pulses and making it possible that only one-way blood flows are imaged.
In cases where, for instance, the head of a subject is imaged with this ASTAR technique, the tagging IR slab is located at an inferior limb side (body""s lower side) position to an imaging slab, while the controlling IR slab is located at a parietal region side (body""s upper side) position. In the present invention, locating the controlling IR slab so that it does not cover the parietal region containing veins is one characteristic that should be maintained. In other words, the controlling IR slab is placed at positions out of the parietal region.
In the ASL method, what should be excluded is normally a signal detected from veins. This xe2x80x9cexclusionxe2x80x9d is, however, in the end, realized if a signal from the veins does not come into an imaging slab during an interval of inversion time (TI). In comparison with the arteries, the veins flow at relatively slower speeds, so that it is not always necessary to apply a controlling slab at a position completely apart from the head. Namely, the position can be determined with which an appropriate margin separated from an imaging slab, depending on flow speeds of veins, the distance of a spatial gap, inversion time, and others.
Incidentally, in the following description, if required, a subject""s region or spatial region at which the controlling slab is located to the imaging slab is simply called xe2x80x9ccontrolling side,xe2x80x9d while a subject""s region or spatial region at which the tagging slab is located to the imaging slab is simply called xe2x80x9ctagging side.xe2x80x9d Further, according to the necessities, images based on echo data acquired by the controlling scan and tagging scan are called xe2x80x9ccontrolling imagexe2x80x9d and xe2x80x9ctagging image,xe2x80x9d respectively.
(Necessary and Sufficient Condition Satisfying ASTAR Technique)
For each of the controlling and tagging sides, let reference signs in FIG. 1 be:
BWcont,BWtag: band widths [Hz] of controlling and tagging IR pulses;
Gcont,Gtag: gradient intensities [Hz/cm] for slab selection at application of IR pulses;
deltaFcont,deltaFtag: offset amounts [Hz] of modulation frequency measured from center of imaging slab;
Offsetcont,Offsettag: distance [cm] between centers of imaging and controlling slabs and distance [cm] between centers of imaging and tagging slabs;
Thickcont,Thicktag: thicknesses [cm] of control and tagging slabs; and
Gapcont,Gaptag: distances [cm] of gaps from imaging slab to each controlling slab of tagging slab,
then the relationships of:
Offsetcontxc2x7Gcont=deltaFcontxe2x80x83xe2x80x83(a)
Thickcontxc2x7Gcont=BWcontxe2x80x83xe2x80x83(b)
Offsettagxc2x7Gtag=deltaFtagxe2x80x83xe2x80x83(c)
Thicktagxc2x7Gtag=BWtagxe2x80x83xe2x80x83(d)
Gapcont=Offsetcontxe2x88x92(Thickimage+Thickcont)/2xe2x80x83xe2x80x83(e)
Gaptag=Offsettagxe2x88x92(Thicktag+Thicktag)/2xe2x80x83xe2x80x83(f)
are realized.
In order to make MT effects caused by the controlling and tagging scans equal in amount to each other, it is necessary and sufficient that the band widths BW and offset frequencies of IR pulses for locating the controlling and tagging IR slabs are equal to each other, respectively, that is,
BWcont=BWtagxe2x80x83xe2x80x83(g)
deltaFcont=deltaFtagxe2x80x83xe2x80x83(h)
are realized.
Thus, from the foregoing expressions (a) to (f),
Offsetcontxc2x7Gcont=Offsettagxc2x7Gtagxe2x80x83xe2x80x83(i)
Thickcontxc2x7Gcont=Thicktagxc2x7Gtagxe2x80x83xe2x80x83(j)
are obtained.
Incidentally, it is required for the ASTAR technique that the IR pulses used for the controlling and tagging scans (controlling and tagging IR pulses) be applied with their application polarities opposite to each other in relation to an imaging slab. For example, if the body axis direction is assigned to the z-axis direction, the center of an imaging slab is determined as the origin, and the upstream and downstream directions of blood flows to be suppressed (for instance, veins) are assigned to the positive and negative, respectively, the expressions of
Offsetcont greater than 0, Offsettag less than 0xe2x80x83xe2x80x83(k)
should be realized. As long as MT effects keep symmetrical with respect to the positive and negative offset excitation frequencies (deltaF) of the IR pulses, both equal absolute values and opposite sings enable those expressions to be realized. But if asymmetrical, the offset excitation frequencies (deltaF), even their signs, should be equal to each other.
Additionally, as stated before, the ASTAR technique requires one more condition that the controlling slab be not located at a parenchyma region containing the veins in the controlling side. Hence, when a limited distance from the z-axis center of an imaging slab to the controlling slab is expressed by Dlimit, the above condition can be met if an expression of:
Offsetcont greater than Dlimit+Thickcont/2xe2x80x83xe2x80x83(1)
is fulfilled.
Therefore, when summarizing the foregoing various conditions in the forms in which the relationships among gradients G, thicknesses Thick, and offsets Offset are included, a necessary and sufficient condition for not only making the amounts of MT effects, caused in an imaging slab when the controlling and tagging scans are performed, equal to each other but also detecting only a signal component from blood flow inflowing from the tagging side can be expressed as follows:                                           Offset            cont                    /                      Offset            tag                          =                              Thick            cont                    /                      Think            tag                                                  =                              Gap            cont                    /                      Gap            tag                                                  =                              G            tag                    /                      G            cont                                                  =                  N          ⁢                      xe2x80x83                    ⁢                      (                          N              ⁢                              :                            ⁢                              xe2x80x83                            ⁢              negative              ⁢                              xe2x80x83                            ⁢              real              ⁢                              xe2x80x83                            ⁢              number                        )                    ⁢                      xe2x80x83                    ⁢          …          ⁢                      xe2x80x83                    ⁢                      (            m            )                              
and
Offsetcont greater than Dlimit+Thickcont/2
Offsettag less than 0xe2x80x83xe2x80x83(n),
in which the controlling side is designated as being positive.
As to the above expression (1), it is an ideal state in which the controlling slab is not completely overlapped on the vein that is not an objective for imaging. On the contrary, this condition is flexible to some extent. If blood to be suppressed flows at a slower speed, it is still enough that the condition is not so strictly established. It is enough only if the blood tagged by the controlling IR pulse does not reach an imaging slab during an inversion time (TI) thereof Thus, it is sufficient to determine the limited distance Dlimit by considering both an organ to be objective and an inversion time thereof. When taking a maximum velocity of blood flow to be suppressed as being v, a practical limited distance Dlimit is expressed by
Dlimit=vxc2x7TIxe2x80x83xe2x80x83(o).
In general, because the flow speed of the vein is lower, it is not required to offset the control slab beyond a necessary value.
(Date Acquisition and Processing Employed by ASTAR Technique)
This ASTAR technique employs, (i) to lessen mis-registration, a way of, called interleaved manner, acquiring data of controlling and tagging images by alternating sequentially in time, shot by shot, the controlling and tagging scans.
Moreover, (ii) in order to obtain images of an imaging slab, difference calculation is performed between controlling and tagging image data. In the present invention, complex-number difference calculation (individual differences for real numbers and imaginary numbers) is executed at the stage of raw data, which is realized by an additional function for processing echo data (i.e., raw data before reconstruction), which is standard-provided in ordinary MRI systems. In other words, when Scont and Stag represent raw data of controlling and tagging images, differences deltaS produced by the complex-number difference calculation is expressed by:
deltaS=|Scontxe2x88x92Stag|xe2x80x83xe2x80x83(p).
Alternatively, such difference calculation may be performed after computation of absolute values, that is,
deltaS=|Scont|xe2x88x92|Stag|xe2x80x83xe2x80x83(q).
In the expressions (p) and (q), it is meant that the processing of absolute values is practically performed after the reconstruction of raw data.
Alternatively, the foregoing difference calculation may be carried out at the stage of image data reconstructed from the raw data.
Furthermore, particularly, (iii) to improve the SNR of a perfusion image, an averaging method is employed, in which each of the controlling and tagging scans is performed a plurality of times to undergo averaging processing.
Where the averaging method is employed and the difference calculation is performed on the above expression (p) (the absolute values are calculated after the difference calculation), differences between the raw data are calculated (complex-number difference calculation), and then added (averaging). This enables continuous data acquisition for each one averaging, shortening the entire time necessary from scanning to data processing. In contrast, in the case of calculating differences based on the above expression (q), such a function of addition cannot be used. In this case, after averaging each of controlling and tagging images, absolute values should be calculated, and then subject to difference calculation.
(Suppression of Signals from Large Blood Vessel in Perfusion Imaging) Needless to say, signals detected from large blood vessels, such as arteries or veins, are indispensable for MRA imaging to observe the blood vessels. However, for perfusion imaging to allow blood capillaries and/or tissue blood to be observed, the signals from those large blood vessels are normally regarded as obstructive signals in the field of clinics.
In the ASTAR technique according to the present invention, the perfusion imaging adopts a way of suppressing signals from the large blood vessels. Specifically, the reconstructed image data deltaV of raw data obtained by the expressions (p) and (q) include signals detected from a large blood vessel (for example, artery) inflowing from the tagging side to an imaging slab. This signals are supposed to be suppressed. When taking an upper limit of a signal from a large blood vessel to be suppressed as deltaVhigh, only a signal component satisfying
deltaV=(deltaV less than deltaVhigh)xe2x80x83xe2x80x83(r)
is extracted. This processing provides perfusion images of which influence of the signal from such a large blood vessel is reduced.
1.2. ASTAR on CASL Technique
On one hand, the ASL imaging according to the present invention can be practiced with the ASTAR based on the CASL (continuous ASL) technique.
In the case of the CASL technique, a continuous wave (CW) having a single frequency that satisfies a given adiabatic condition is continuously applied to the inflow-side portion of an artery for more than a certain period. This application causes spins in the blood flow to be inverted, which then inflow into a downstream imaging slab.
Imaging on this CASL technique is provided with two ways: (1) one way is to use a small transmitting RF coil having no sensitivity on an imaging slab, where, without applying a gradient, the RF coil is such excited that the sensitivity region thereof includes inflowing arteries such as carotid arteries (for instance, refer to xe2x80x9cMRM 33, 209-214 (1995)xe2x80x9d); and (2) the other way is to use an ordinary head-dedicated RF coil to apply a continuos wave together with a gradient (for instance, refer to xe2x80x9cRadiology 1998; 208: 410-416xe2x80x9d). In the latter, if the continues wave is applied concurrently with a gradient in the cephalocaudal direction (Z-axis direction), spins residing in a thin tagging slice (theoretically, a plate, but refereed to as a slab, for the sake of convenience) nearly perpendicular to blood flows such as arteries are excited, as shown in FIG. 18, and then the blood flows of which spins are inverted through the slice flow into an imaging slab.
Although the PASL technique requires that spins existing in a slab of a certain thickness be inverted, actual slabs (tagging and controlling slabs) in which spin inversion occurs become extremely thin, as shown in FIG. 18. Hence, under the PASL technique, blood present at an inflowing side in the tagging slab, which is far from the imaging slab, takes more time to reach the imaging slab, during which time the T1 relaxation of the spins proceeds to a greater extent, resulting in that the SNR of blood flow images is lowered. However, the CASL technique is able to lessen the influence of this problem caused by the inflowing delay.
In the case of the CASL technique, like the PASL technique, imaging of only blood flows with the use of the RF coil above-described in either way (1) or (2) involves difference calculation between two images of which objective blood flows are not tagged (not inverted) (controlling mode) and tagged (tagging mode). This permits signals from stationary tissue to be cancelled. If using the small RF coil described in the above way (1), differences in signal caused by MT effects can be almost ignored, as long as the sensitivity region of the coil is shifted from the imaging slab. On the contrary, when the head-dedicated RF coil described in the above way (2) is used and moreover, a transmitting sensitivity in RF application to the tagging slab covers a region including an imaging slab, it is needed that MT effects influencing the imaging slab be canceled. Namely, the MT effects in the controlling mode should be controlled to the same amount as that in the tagging mode. Even in this case, like the PASL technique, it is essential that one of arteries or veins, for example, veins, be suppressed,so that they are not depicted.
Where the ASTAR technique is performed on the CASL technique involving the use of the head-dedicated RF coil described in the above way (2), it is considered that the excited slabs (tagging and controlling slabs) by the PASL technique are equivalently changed into thin plates (refer to FIGS. 1 and 18).
In applying the CASL technique to the EPISTAR later described, because setting of Offsettag=Offsetcont is indispensable, it is impossible to suppress the depiction of veins inflowing from the controlling side. Hence practicing the ASTAR on the CASL technique, by which a state of Offsettag less than Offsetcont can be established, is effective, because the advantages of both of them can be obtained.
2. Approach on EPISTAR Technique
An approach on the EPISTAR technique will now be described. This approach is based on post-processing of signals acquired by scanning on the EPISTAR technique, which accomplishes the object of the present invention.
2.1. Depiction of Blood Flow in One Direction on Signal Processing
FIG. 2 pictorially shows slabs spatially located on basis of the EPISTAR technique under the similar dimensions to those in FIG. 1
Based on the EPISTAR technique, as described before, canceling differences in MT effects needs both controlling and tagging slabs symmetrically located with respect to an imaging slab, the controlling slab having an identical (symmetrical) slab thickness and a distant offset amount with those of the tagging slab. When calculating differences between as-acquired controlling and tagging images with an ordinary ASL technique, blood flows inflowing into the imaging slab in the two inflow directions are both imaged.
Therefore, in the present invention, with which the symmetry of the slab thicknesses and distance offset amounts is still mainlined, which are characteristic of the EPISTAR technique, scanning is performed to acquire data of controlling and tagging images, and a characteristic processing is such performed that only a signal component corresponding to a desired blood flow is extracted in the stage of processing the data for visualization.
Let assume that raw data (complex data) acquired by tagging and controlling scans are Scont and Stag, respectively, and their reconstructed image data are Vcont and Vtag, respectively. If the signal processing for these image data includes calculating differences, before calculating absolute values, that is,
deltaV=|Vcontxe2x88x92Vtag|xe2x80x83xe2x80x83(s)
is calculated, the signals of blood flows inflowing into an image slab in both of the directions are mixed with an image thereof. Instead of this, calculating the absolute values of the reconstructed image data Vcont and Vtag before the difference calculation, that is,
deltaV=|Vtag|xe2x88x92|Vcont|xe2x80x83xe2x80x83(t)
may be performed. Thus, with respect to a signal component of blood inflowing from the controlling side, it is true that
deltaV less than 0xe2x80x83xe2x80x83(u),
while, concerning a signal component of blood inflowing from the tagging side,
deltaV greater than 0xe2x80x83xe2x80x83(v)
is established. Thus, obtaining data components deltaV fulfilling this expression (v) enables blood inflowing from the tagging side (normally, assigned to arteries) to be extracted separately.
(Perfusion Image)
As described before, compared to large blood vessels, such as arteries and veins, signals acquired from flows of thin blood vessels, such as blood capillaries and blood flows in tissue, are considerably lower in intensity. In addition, their flowing directions are not restricted to one, and those types of blood are supposed to flow into each voxel composing an imaging slab from every direction. Hence simply extracting signal components satisfying deltaV greater than 0 from the data deltaV calculated with the expression (t) results in that perfusion components on blood inflowing from the controlling side are suppressed, and are difficult to reflect on an perfusion image.
In perfusion imaging, what should be suppressed from a clinical point of view are signals detected from large blood vessels (arteries and veins). Thus, signal processing is performed such that perfusion components on blood inflowing into an imaging slab from both of the directions remain on a perfusion image and only signals from large blood vessels are suppressed thereon. In the present invention, this processing is done with thresholds. Practically, for the above expression (t) of:
deltaV=|Vtag|xe2x88x92|Vcont|,
only signal components meeting an expression of:
deltaV=(xe2x88x92deltaVlow less than deltaV less than deltaVhigh)xe2x80x83xe2x80x83(w),
where deltaVlow:upper limit of perfusion signals
deltaVhigh:lower limit of large blood signals
are extracted.
In order to accomplish the foregoing object, the present invention adopts the following configurations based on the foregoing principles.
First, according to an approach on the ASTAR technique, there is provided an MRI system for obtaining an ASL (Arterial Spin Labeling) image of an imaging slab of an subject placed in a static magnetic field by locating a tagging slab and a controlling slab at one side and the other side of the imaging slab, the system comprising: setting means for setting a first RF wave and a first magnetic gradient both for selective-exciting the tagging slab and a second RF wave and a second magnetic gradient both for selective-exciting the controlling slab, in which offset amounts of exciting central frequencies of both the first and second RF waves to a central position of the imaging slab are equal to each other and offset positions of the tagging slab and the controlling slab to the imaging slab are different from each other; a first scanning means for performing a pulse sequence including he first RF wave and the first magnetic gradient so as to acquire a first MR signal from the imaging slab; a second scanning means for performing a pulse sequence including the second RF wave and the second magnetic gradient so as to acquire a second MR signal from the imaging slab; image data producing means for producing image data based on mutual difference between the first and second MR signals; and visualizing means for visualizing the image data as an ASL image.
A preferred one example is that each of the first and second RF waves consists of a single-frequency continuous RF wave determined correspondingly to each of the first and second magnetic gradients so as to excite a desired slab position.
Another preferred example is that the setting means set conditions concerning the first and second RF waves and the first and second magnetic gradients such that a ratio between slab thicknesses of the tagging and controlling slabs and a further ratio between the positional offsets of the tagging and controlling slabs to the imaging slab are equal to each other. In this case, for example, each of the first and second RF waves consist of a pulsed wave having a certain frequency band.
In the above configurations, each of the first and second RF waves may be an IR wave inverting spins, and the first and second scanning means may be configured to apply the IR pulse in mutually opposite polarities to the imaging slab. In this case, preferably, the imaging slab is located at a head portion of the subject, and the setting means have means for setting the controlling slab separately from the head portion.
Moreover, in each configuration stated above, the image data producing means may comprise means for extracting from a difference of the MR signal a signal component of not more than a threshold determined as a minimum signal intensity for a blood vessel to be suppressed of the subject. This provides a perfusion image.
Further, in each configuration stated above, it is preferred that each of the first and second scanning means include means for performing the same type of pulse sequence enhancing a longitudinal magnetization of the spins of the subject as well as including each of the first and second RF waves. For example, each of the first and second RF waves is an IR wave for inverting spins, the IR wave being applied slice-selectively. A preferred example is that each of the pulse sequences includes a pre-saturation pulse for previously saturating spins of the subject.
Further, in each configuration stated above, the first and second scanning means may comprise means for performing the pulse sequences every application of the RF waves in an interleave system.
Still further, in each configuration stated above, each of the first and second scanning means may be configured to perform a plurality of times the acquisition of the MR signal from the imaging slab, and the image data producing means may include means for averaging the MR signal acquired the plurality of times. This is able to improve an SNR.
In the foregoing configurations according to the PASL technique, the setting means is able to comprises means for providing as known amounts a slab thickness of the imaging slab, a slab thickness of the tagging slab, a distance between the imaging and tagging slabs, and a distance between the imaging and controlling slabs; and calculating means for calculating a slab thickness and a positional offset amount of the controlling slab on the basis of the known amounts.
Still, according to a preferred example, there is provided a pulse sequence in which, between application of each of the first and second RF waves and application of an imaging pulse train, a non-slice selective IR wave to be applied to a region of the subject including the imaging slab, tagging slab, and controlling slab is placed. In this configuration, preferably, an interval from application of the non-slice selective IR pulse to application of the imaging pulse train in the pulse sequence can be determined so that a spin-lattice relaxation time of stationary tissue contained in the imaging slab becomes an amount regarded as being approximately zero on average at a time of applying the imaging pulse train. This is able to steadily reduce difference errors specific to the stationary tissue, and provide an ASL image in which blood flow is mainly depicted. For example, the non-slice selective IR wave consists of a plurality of non-slice selective IR waves. Of course, one non-slice selective IR wave may be applied.
On one hand, according to an approach on the EPISTAR technique, provided is an MRI system comprising: a first scanning means for applying a first RF wave to a tagging slab to be located in one side of an imaging slab of a subject so as to acquire a first MR signal from the imaging slab; a second scanning means for applying a second RF wave to a controlling slab to be located, symmetrically to the tagging slab, in the other side of the imaging slab so as to acquire a second MR signal from the imaging slab; and image data producing means for producing image data on the basis of the first and second MR signals, wherein the image data producing means comprise a first and second absolute-value calculating means for calculating absolute values of the first and second MR signals after reconstruction thereof, respectively; difference means for performing mutual differences between the absolute values of the first and second MR signals; and extracting means for extracting image data of a desired signal component from differences obtained by the difference means.
In this configuration, for example, the extracting means are configured to make the differences obtained by the difference means have threshold processing with a threshold determined to suppress a signal of a large blood vessel inflowing from a controlling slab side into the imaging slab, so that a signal component of a blood flow inflowing from a tagging slab side into the imaging slab is extracted. Further, the extracting means may be configured to make the differences obtained by the difference means have threshold processing with a threshold determined to suppress a signal of a large blood vessel inflowing into the imaging slab, so that a signal component of perfusion inflowing into the imaging slab is extracted.
Furthermore, according to the ASTAR technique of the present invention, provided is a n MR imaging method of obtaining an ASL (Arterial Spin Labeling) image of an imaging slab of an subject placed in a static magnetic field by locating a tagging slab and a controlling slab at one side and the other side of the imaging slab, comprising the steps of: setting a first RF wave and a first magnetic gradient both for selective-exciting the tagging slab and a second RF wave and a second magnetic gradient both for selective-exciting the controlling slab, in which offset amounts of exciting central frequencies of both the first and second RF waves to a central position of the imaging slab are equal to each other and offset positions of the tagging slab and the controlling slab to the imaging slab are different from each other; performing both a pulse sequence including the first RF wave and the first magnetic gradient so as to acquire a first MR signal from the imaging slab a pulse sequence including the second RF wave and the second magnetic gradient so as to acquire a second MR signal from the imaging slab; producing image data based on mutual difference between the first and second MR signals; and visualizing the image data as an ASL image.